# CASING DESIGN - SURFACE CASING LOADS DESIGN

## SURFACE CASING LOAD CASES AND DESIGN

Design loading conditions for surface casing are illustrated in below drawing for burstcollapse, and tension considerations.

The max internal pressure used for the Burst design is based on a Well Control condition assumed to occur while circulating out a large kick.

The max external pressure used for the Collapse design is based on a severe lost circulation problem.

The max axial Tension  loading condition is based on assumption of stuck casing while the casing is run in the hole before cementing operations.

### Casing Burst Design

The burst design should insure that formation fracture pressure at the casing shoe will be exceeded before the casing burst pressure is reached. Therefore, this design uses formation fracture as a safety pressure release mechanism to assure that casing rupture will not occur at the surface and endanger lives.

The design pressure at the casing seat =  Fracture Pressure + Safety Margin.
So that to allow for an injection pressure that is slightly higher than the fracture pressure.  If the fracture gradient is not known, a gradient of 1.0 psi/ ft may be safely assumed.

The Casing Surface Pressure = Bottom Hole Fracture Pressure + Safety Margin - Hydrostatic Pressure of the gas column.

• The pressure inside the casing is calculated  assuming that all of the drilling fluid in the casing is lost to the fractured formation, leaving only formation gas in the casing.
• Assume a minimum gas gradient of 0.10 psi/ft for pressures originally shallower than 10,000 ft and 0.15 psi/ft for pressure sources deeper than 10,000 ft
• If the formations below the surface casing do not have any gas, then gradients of the formation fluids (oil or water) should be used

The External Pressure = Normal Formation Pore Pressure

• The external pressure, or back-up pressure outside the casing that helps resist burst, is assumed to be equal to the normal formation pore pressure.
• The beneficial effect of cement or higher  density mud outside the casing is ignored because of the possibility of both a locally poor cement bond and mud degradation that occur over time.
• A safety factor of 1.1 is used to provide an  additional safety margin during transportation and handling of the pipe.

The burst Load at the casing seat = Fracture Pressure + Safety Margin - Formation Pore Pressure

The burst Load at the surface = Surface Pressure inside the casing.

#### The Burst Load Design Line

The burst load line is defined by two points; burst load at the casing seat and the burst load at the surface. Connecting the two points gives the burst load line in the casing from top to bottom.
Multiplying the burst loads at the two points by a safety factor determines the burst design line.

### Casing Collapse Design

The collapse design is based on the most severe lost-circulation problem that is felt to be possible or on the most severe collapse loading anticipated when the casing is run.

Now lets consider the following:
• The maximum possible external pressure that tends to cause casing collapse results from the fluid that is in the hole when the casing is placed and cemented.
• The beneficial effect of the cement and of possible mud degradation is ignored
• The detrimental effect of axial tension on collapse-pressure rating is considered.
• The collapse rating should be de-rated above the neutral point using below equation.

• Below the neutral point the casing is in compression and adjustment of the collapse rating is not required. The depth of the neutral point of a casing string in mud can be calculated by the following formula:
Dn=D( 1 - w/489 )
D= depth to neutral point, ft
Dt  = setting depth of casing string, ft
W  = mud weight, pcf
When correcting the collapse-pressure rating of the casing, it is recommended that the axial tension be computed as the hanging weight of the casing for the hydrostatic pressures present when the maximum collapse load is encountered plus any additional tension put in the pipe during and after landing.

The beneficial effect of pressure inside casing can also be taken into account by the consideration of a maximum possible depression of the mud level inside the casing. A safety factor generally is applied to the design-loading condition to provide an additional safety margin. The minimum fluid level in the casing while it is placed in the well depends on field practices.

The casing usually is filled with mud after each joint of casing is made up and run in the hole, and an internal casing pressure that is roughly equivalent to the external casing pressure is maintained. However, in some cases the casing is floated in or run in at least partially empty to reduce the maximum hook load before reaching bottom. If this practice is  anticipated, the maximum depth of the mud level in the casing must be used in collapse calculations.

### Casing Tension Design

Tension design requires a consideration of axial stress present when the casing is run, during cementing operations, when the casing is loaded in the slips, and during subsequent drilling and production operations.

The design load is usually based on conditions that occur when the casing is run such as casing stuck.

• It is assumed that the casing becomes stuck near the bottom of the hole and that a minimum amount of pull, in excess of the casing weight in mud, is required to pull the casing free.
• A minimum safety factor criterion is applied such that the design load will be dictated by the maximum load resulting from the use of either the safety factor or the overpull force whichever is greater.
• The minimum overpull force tends to control the design in the upper portion of the casing string, and the  minimum safety factor tends to control the lower part of the casing string.

### Summary

Once the casing design is completed, maximum axial stress anticipated during cementing, casing landing, and subsequent drilling operations should also be checked to ensure that the design load is never exceeded.

When the selection of casing weight and grade in a combination string is determined by collapse, a simultaneous design for collapse and tension is best. The greatest depth at which the next most economical casing can be used depends on its corrected collapse pressure rating, which in turn depends on the axial tension at that depth. Therefore, the corrected collapse-pressure rating cannot be computed until the axial tension is calculated.

It takes an iterative procedure, in which the depth of the bottom of the next most economical casing section is first selected on the basis of uncorrected table value of collapse resistance, to be used. The axial tensionat this point is then calculated, and the collapse resistance is then corrected. This allows
the depth of the bottom of the next casing section to be updated for a second iteration. Several iterations may be required to arrive at a solution.